The focal surfaces typically found in optical systems such as optical spectrometers, optical multiplexers, optical demultiplexers, etc. usually have a curved shape of some type. For simplicity, the focal surface may also be referred to as a focal plane or a focal curve. The focal surface may be a simple surface, for example, a sphere, or may have a complex shape with intricate dimensions. In refraction based systems which may utilize a lens, the focal curve may be referred to as what is known in the art as Petzval field curvature. In grating based systems, the focal curve may take the shape of the commonly known “Rowland Circle.” The Design of Optical Spectrometers, by J. F. James and R. S. Sternberg, (Chapman and Hill, 1969) describes a Rowland Circle as follows:                “If a plane is taken through the center of the grating, perpendicular to the rulings, a circle can be drawn, touching the grating at its center and with a diameter equal to the radius of curvature of the grating. This circle is called Rowland circle. The focusing property of the grating is such that a source of light placed at a point on the Rowland circle is diffracted and refocused to form an image of the source at some other point on the Rowland circle.”        
As an example, FIG. 1 shows a plot of variations of diffracted focal distance as a function of wavelength for a system employing a diffraction grating. FIG. 1 shows variations for several values of the coefficient Cf, related to the spectral dispersion as a function of wavelength. The curve for Cf=0 shows a relatively simple focal curve with the focal distance, or the diffracted image distance, decreasing as the diffracted wavelength increases. The curve for Cf=0.22 shows a complex curve, where the focal distance varies both positively and negatively as the diffracted wavelength increases.
FIG. 2 shows a plot of several spectral focus planes for a concave grating for various values of the coefficient Cf.
When detecting optical radiation having a focal surface, detection devices are ideally placed directly on the focal surface to minimize losses. These losses may include optical energy losses and/or resolution losses due to unfocused optical radiation impinging on the detector devices. Semiconductor components are frequently used as optical detector elements. These semiconductor components may typically include a quasi-linear or flat detection surface, e.g. a photodiode line or a CCD sensor.
However, placing a detection device on the focal plane of an optical system may present some difficulties. For example, when the detection devices are arranged in a quasi linear fashion, such as a photodiode line, or when the devices are arranged in a two dimensional, flat planar fashion, such as a CCD array, the detection devices cannot reside on a curved focal surface. For example, as mentioned above, the focal surface may be described by a Rowland circle. However, an attempt to place a linear or planar array of detectors on the Rowland circle results in the optical spectrum being in focus at only two wavelength positions, that is, those positions where the Rowland circle intersects the photodiode line or the planar array. At other wavelengths, the optical radiation is not in focus, resulting in at least poor resolution and/or energy transfer. As a further example, currently known optical demultiplexers may receive polychromatic light through various mechanisms, including fiber optics, and that polychromatic light according to its spectrum. The beams of the various wavelengths may be dispersed at various angles with focal points having varying distances from the dispersive element. These focal points may form a focal curve which may be imaged by photosensitive elements. However, if a straight line of elements, such as a commonly available photodiode line, is used to image the focal curve, only the elements that intersect the focal curve will detect the correct amount of optical energy.
When holographic concave gratings are used, the variable groove dimensions allow imaging errors to be minimized to systems stigmatic at three wavelengths. Reference in this regard may be had to “Diffraction Gratings Ruled and Holographic”, Handbook, Jobin Yvon S. A.
As a result, for systems using linear, quasi-linear, or planar detection devices, the optical resolution and/or detected energy usually remains attenuated or substantially impaired over a significant part of the spectral range being utilized.
A method is presented in U.S. Pat. No. 4,467,361, entitled “Image Pick-Up Apparatus,” by Ohno et al., issued Aug. 21, 1984, where the curved focal surface is coupled to a surface of a photosensor by means of light guides of different lengths. This method has various disadvantages, however, including light loss from unused surfaces between the fiber end surfaces, light loss from reflections off the end surfaces of the fibers, light loss from the finite transmission of the fiber material, photodegradation of the fiber material, and limited resolution of the finite fiber diameters.
EP 768 814 discloses three-dimensional photosensitive structures for optical detectors. These three-dimensional structures are created by applying layers and/or etching recesses. One disadvantage is that special and expensive processes are required to manufacture the photosensitive semiconductor structures. Another disadvantage is that the dimensions of the three-dimensional structure must be known at the time of manufacture.
An additional disadvantage of known optical systems is their focal lengths. Advances in technology and optical component quality have resulted in optical systems with decreasing focal lengths over time. Currently available small gratings and miniaturized spectrometers exhibit focal lengths as small as approximately 20 mm. However, even shorter focal lengths may be achieved if optical detection systems could be constructed with a radius of curvature that would allow for a focal length of, for example, on the order of 10 mm. For example, in an optical system based on a Rowland Circle, the focal length l is determined by the relationship l=R*cos A, where R is the radius of curvature of the grating and A is the angle between the grating normal and a straight line from the grating center to the focal point. Therefore if the angle A remains constant, a decrease in the radius of curvature results in a decreased focal length.
Accordingly, it is an object of this invention to provide an optical device that may be adapted to a focal surface of an optical system so as to efficiently couple optical energy from the optical system to the optical device.
It is another object of this invention to provide an optical device that may be adapted to a plurality of different focal surfaces.
It is still another object of this invention to provide an optical device having a flexible, thin structure which may be easily formed to the shape of a plurality of different focal surfaces.
It is a further object of this invention to provide an optical device which may attain a radius of curvature that allows for an optical system with a focal length that is substantially shorter than the current state of the art.